Surface force

Surface force denoted f_s is the force that acts across an internal or external surface element in a material body. Normal forces and shear forces between objects are types of surface force. All cohesive forces and contact forces between objects are considered as surface forces. Surface force can be decomposed into two perpendicular components: normal forces and shear forces. A normal force acts normally over an area and a shear force acts tangentially over an area.

Block on a ramp and corresponding free body diagram of the block showing the surface force from the ramp onto the bottom of the block and separated into two components, a normal force N and a frictional shear force f, along with the body force of gravity mg acting at the center of mass.

Equations for surface force

Surface force due to pressure

${\displaystyle f_{s}=p\cdot A\ }$, where f = force, p = pressure, and A = area on which a uniform pressure acts

Examples

Pressure related surface force

Since pressure is ${\displaystyle {\frac {\mathit {force}}{\mathit {area}}}=\mathrm {\frac {N}{m^{2}}} }$,^[1] and area is a ${\displaystyle (length)\cdot (width)=\mathrm {m\cdot m} =\mathrm {m^{2}} }$,

a pressure of ${\displaystyle 5\ \mathrm {\frac {N}{m^{2}}} =5\ \mathrm {Pa} }$ over an area of ${\displaystyle 20\ \mathrm {m^{2}} }$ will produce a surface force of ${\displaystyle (5\ \mathrm {Pa} )\cdot (20\ \mathrm {m^{2}} )=100\ \mathrm {N} }$.

See also

• Body force
• Contact force